Switching audio amplifiers achieve high power efficiency by using a special (switching) output stage, which outputs pulses that switch between the positive power supply voltage and negative power supply voltage of the output stage. To produce the pulses, the output stage contains switches to connect its output to the desired power supply. The pulse waveform minimizes v*i power dissipation across the switches, resulting in significantly less power consumption than ‘linear’ output stages, which can produce any voltage between the power supply levels, but at the cost of larger voltage drops and power consumption in the output-producing circuit elements.
In a switching audio amplifier, a modulator converts an input audio signal into a pulse sequence for use in the switching output stage. Typical modulators replicate the desired audio frequencies in the spectrum of the output pulses, with techniques like varying the pulse width in proportion to the input audio signal (pulse-width modulation, or PWM), or using pulses with unit width but varying their density in the pulse sequence in proportion to the input audio signal (pulse-density modulation, or PDM). However, the pulses also contain undesirable high-frequency energy, which can interfere with other electronics, or lead to undesirable noises or heating if input directly to the speaker. Consequently, a low-pass ‘demodulation’ filter is often inserted between the switching output stage and speaker, to remove high-frequency noise while passing desired audio frequencies. Power dissipation in the filter elements must be minimal to retain high power efficiency overall for the amplifier system, and so a passive, LC filter is generally used. There are many possible modulation schemes, and either single-ended or differential amplifier implementations are possible.
A troublesome design problem in differential switching PWM amplifiers with demodulation filters is how to avoid undesirable clicks and pops (audible noise) when turning them on or off, muting or un-muting and the like. When the amplifier is completely off, demodulation filter capacitors are discharged, and the output stage is not switching (no pulses). This is very different from the zero-output condition when the amplifier is on. Here, the capacitors in the demodulation filter are charged and the output stage is producing the PWM representation of 0 signal, which is pulses with 50% duty cycle. In conventional 2-level differential modulation, there are + and − pulse signals which have 180 degrees phase difference. The challenge is to somehow accomplish a transition between these dissimilar operating states without producing objectionable audible noise, clicks or pops, at the speaker.
A traditional prior art solution is to insert a relay between each speaker terminal and the demodulation filter. The relay is kept open during amplifier turn-on or turn-off, and is only closed after any undesirable transients have settled. This prevents problematic transients from reaching the speaker, but has disadvantages of extra cost and space. A second prior art solution is to start or stop modulation only when the positive power supply to the output stage is low. When the modulation is on, the power supply voltage is slowly ramped to or from the desired operating value depending on whether the modulation is being started or stopped. This approach requires a special power supply which is capable of being ramped up and down, significantly increasing complexity and cost. See U.S. Pat. No. 6,720,825.
More recent prior art solutions overcome the disadvantages of the first two by using special pulse sequences to minimize energy at audio frequencies during transitions between the OFF and ZERO-SWITCHING states. U.S. Pat. Nos. 6,384,678 and 6,720,825).